The overall objective of the proposed studies will be to define and characterize developmental changes in the mouse mammary epithelium associated with multiple hormonal induction. The developing mammary ductal epithelium is structurally and functionally heterogeneous: there are three physiologically distinct subpopulations of epithelial cells, ductal epithelial, alveolar (secretory) epithelial and myoepithelial. The specific aims will be to: 1) define, purify and characterize mammary epithelial membrane markers that may distinguish between the three subpopulations of mammary epithelial cells. 2) define the interrelationship of mammary epithelial differentiation markers with other normal cell surface molecules (i.e. Ia, H-2 antigens) and define the cellular localization of markers in mammary gland using immunohistochemistry. Mammary tissue will be obtained from all developmental stages of mammary gland (i.e. virgin greater than pregnant greater than lactating greater than involuting). Dissociated cells released by enzyme treatment of the mammary tissue will be fractionated on Ficoll and Percoll density gradients. Subpopulations of mammary epithelial cells will be radiolabeled (125I surface labeling, 3H-fucose, 3H-leucine and 35S-methionine metabolic labeling) and standard biochemical techniques of detergent cell lysis and limited surface proteolysis will be used to obtain extracts. Mammary cell extracts will be analyzed and purified using SDS-PAGE immunoprecipitation and immunoadsorbent chromatography. The long-term objectives will be to produce monospecific and monoclonal antibodies to defined murine mammary epithelial differentiation markers. In the neonatal period the young animal requires maternal immunological protection until its own mechanisms become competent. During late pregnancy and lactation, the multiple mammotropic hormones stimulate differentiation of the mammary glandular epithelium and activation of the local secretory immune system as evidenced by an increase in the number of infiltrating IgA plasmablasts from gut-associated lymphoid tissues. The studies of secretory mammary epithelial cell membranes may yield information on the receptors that play a role in the interaction between epithelial cells and IgA plasmablasts and the transepithelial translocation and exocytosis of secretory IgA (IgA dimer from plasma cells complexed with secretory component from epithelial cells).